Why is force not on a scalar quantity?

Because force always has a direction, it always works towards or against something.

you might know that force,

is rate of change of momentum i. e

force = m (v-u) / t

= (mv - mu) / t

as we know momentum is a vector quantity so, the rate of change of momentum i. e Force would also be a vector quantity.

momentum = mass * velocity

velocity has a direction so,

momentum has also got a direction.

so, momentum is also a vector quantity.